Regis Philbin and David Letterman Discuss Returning to TV After Sept. 11 on CNN (Video)

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Pinch-hitting for Piers Morgan, the former "Live" host broaches several more serious subjects with his "Late Show" friend in a rare interview.

If CNN is making a play for more viewers in the wake of recent record-low ratings, it may likely win a few extra eyeballs on Tuesday night when Regis Philbin stages a temporary return to hosting after leaving Live back in November 2011.

Vacationing Tonight host Piers Morgan's empty seat is being occupied by a string of guests this week and Tuesday's episode finds Philbin conducting a rare televised sit-down with Late Show host and longtime friend David Letterman.

Among the topics covered by the men -- retirement, fatherhood, Johnny Carson -- Philbin brings up joining Letterman for his solemn first broadcast after the Sept. 11 terrorist attacks.

"I remember that night, and I remember not wanting to go back, not feeling ready to go back -- but knowing we had to go back," Letterman tells Philbin. "I was trepidatious about it because I didn't know what to say and didn't know what to do, but I knew if you were there, we could make something of that."

The broadcast found the duo, joined by Dan Rather, for an uncharacteristically dry episode that found Letterman offering kind words for New York firefighters, police officers and the mayor at the time, Rudy Giuliani.

"There was nothing I initiated," Letterman adds. "It was all me reacting."

The full interview airs at Tuesday at 9 p.m. ET, on CNN. Watch the clip below: